This invention relates generally to varactor diodes. More specifically, the invention pertains to varactor diodes for frequency multipliers at, and above, submillimeter-wave frequencies.
An electronic circuit that produces an output frequency that is an integral multiple of the input frequency is known as a frequency multiplier. One type of frequency multiplier uses the nonlinear capacitance of a semiconductor PN junction diode to couple energy from an input circuit that is tuned to a fundamental input frequency to an output circuit, which is tuned to a desired harmonic of the input frequency. A semiconductor junction device that has a nonlinear capacitance is known as a varactor. The basic physics of the varactor is described in S. M. Sze, Physics of Semiconductor Devices, 2nd Edition, Wiley 1981, pp. 114-116.
In essence, a varactor diode is a diode that can behave as a capacitor in the presence of a reverse voltage. When a reverse voltage is applied to a PN junction, the holes in the p-region are attracted to the anode terminal and electrons in the n-region are attracted to the cathode terminal. The resulting depletion region, between the anode and the cathode, is substantially devoid of carriers, and behaves as the dielectric of a capacitor.
The depletion region widens as the reverse voltage across it increases; and since capacitance is inversely proportional to dielectric thickness, the capacitance will decrease as the reverse voltage across the PN junction increases. Therefore by varying the reverse voltage across a PN junction the junction capacitance can be varied. Variations in reverse voltage have a non-linear effect on capacitance. It is this nonlinearity that allows the varactor to be used as a harmonic generator.
Frequency multipliers that provide consistent and dependable power at submillimeter wave frequencies are highly desirable for a variety of applications including; space and upper atmosphere imaging, sensing and communication applications. Terahertz satellite communication links can also support data transfer rates exceeding the capabilities of existing microwave systems. Thus a growing number of millimeter-wave applications has created a need for local-oscillator sources operating at terahertz frequencies. T. W. Crowe and T. C. Grein at the University of Virginia (Charlottesville, Va.) and R. Zimmermann and P. Zimmermann at Radiometer Physics GmbH (Meckenheim, Germany) disclose a tripler design that utilizes whisker-contacted, Schottky-barrier varactor (SBV) diodes to deliver an 800-GHz output frequency. See Progress Toward Solid-State Local Oscillators at 1 THz, IEEE Microwave and Guided Wave Letters, Vol. 6, No. May 5, 1996, p. 207, 1996. Also see E. Kollberg and A Rydberg, Quantum Barrier Varactor Diode For High Efficiency Millimeter- Wave Multipliers, Electron Lett. Vol. 25, p. 1969, 1989: Demonstrating GaAs/AlGaAs heterostructure barrier varactor with symmetric C-V profile. The type of device referred to in this article suffers form high leakage current due to a low barrier height. Additionally R. Havart, E. Lheurette, O. Vanbesien, P. Mounaix, F. Mollot, and D. Lippens, in Step-like Heterostructure Barrier Varactor, IEEE Trans. Elect. Dev. Vol. 45, p. 2291, 1988 report that AlInAs/AlAs/AlInAs heterostructure barrier has an improved leakage current.
Therefore, an object of the present invention is to provide a method and apparatus that improves the efficiency of a frequency multiplier so as to decrease the power loss arising during the frequency multiplication.
This invention provides a novel varactor diode for frequency multipliers at submillimeter wave frequencies and above. Functionally, this device replaces the conventional heterostructure barrier varactor. The two essential features of this Sb-based quantum well heterostructure barrier varactor are (1) the AlSb/AlAsSb heterostructure barrier and (2) bandgap engineered triangular quantum well cathode and anode.